The invention concerns a fluid friction clutch, in particular for a fan for the radiator of an internal combustion engine.
A fluid friction clutch of this type is known from U.S. Pat. No. 4,305,491. The fluid friction clutch, frequently referred to as a viscous clutch, serves to drive a fan for the radiator of an internal combustion engine. The clutch is controlled by a temperature sensor. For this there is employed a solenoid or lifting magnet, which operates a valve element via a piston rod, which valve controls the supply of a viscous fluid in a work space, wherein this fluid effects a frictional engagement between the drive side and the driven side of the clutch mechanism.
The operation of the valve element by the piston rod, which is positioned coaxially within the drive shaft, makes it possible to place the solenoid and the control connections therefore upon the backside of the clutch and clutch driven fan. Thereby a space-saving and easy-to-assemble construction of the fan and clutch is made possible.
In the known fluid friction clutch, the entire solenoid mechanism as well as the piston rod operated thereby is integrated in the rotating drive shaft. It is thus necessary to introduce the activation current into the actuator coil, which is rotating along with the drive shaft, via a frictional contact, which contact is liable to frictional wear.
The invention is thus based upon the task of improving the fluid friction clutch of the above mentioned type, such that friction contacts liable to wear can be avoided.
In accordance with the invention, the anchor via which the piston rod is operated is guided coaxially directly in the hollow drive shaft. The armature coil, in contrast, is not integrated in the drive shaft, but rather is seated within a special magnet body, which coaxially surrounds the drive shaft and in which the drive shaft is rotatable. In this manner the magnet body can be fixedly assembled with the armature coil, so that no friction contacts are necessary for introduction of current into the armature coil. The drive shaft, at least in the part guiding and surrounding the anchor, is comprised of a soft magnetic material, wherein a separating gap of a non-magnetic material accomplishes that the magnet flow is not short-circuited in the drive shaft, but rather is guided through the anchor. Likewise, the magnet body is comprised of a soft magnetic material, so that the magnetic flow of the armature coil is guided through the magnet body, the area of the drive shaft surrounding the anchor, and the anchor. The fixed construction of the armature coil simplifies the construction and assembly and reduces the likelihood of defects.
The solenoid control can be so designed, that the anchor is moved between two end-positions. In such a so-called ON-OFF-control the clutch, and therewith the fan, are timewise activated in cycles. If the cycle frequency exceeds a boundary or limiting frequency as determined by the capability of the system (mass moment of inertia and viscosity of the fluid), then the anchor and the valve element can not carry out a complete stroke cycle within the period of the switch cycle, and the anchor and the valve element become positioned or set to an intermediate position in the stroke path. Thus, the position of the anchor along the stroke path, and therewith the degree of opening of the valve element, can be controlled via the cycle frequency, so that a continuous clutch engagement can be achieved.
In another embodiment the solenoid is controlled as a proportional solenoid. The hub position of the anchor can thereby be continuously adjusted within the total hub stroke length by the equilibrium of the forces acting upon the anchor, namely, on the one hand, the magnetic force determined by the energizing current and on the other hand the return spring force. By controlling the excitation current strength and the increasing entry of the anchor into the ferromagnetic area of the drive shaft, the continuous engagement of the clutch can be controlled in this embodiment.
Other benefits, features, and advantages of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanying drawings and appended claims.